With these devices it is also possible to treat such mixtures containing different plastic material, wherein a certain kind of thermoplastic in the mixture can be selectively softened, and the mixture can subsequently be passed along in a softened and heated state to the following processing station.
From WO 93 14915 and the DE 42 20 665 processes are known wherein the relaxation of stretched thermoplastics for recycling purposes is performed so that within a mixed flow certain thermoplastics are selectively relaxed corresponding to type causing selectively targeted changes of their characteristics and making possible a subsequent automatic sorting. From WO 94/00241 and WO 93/17852 processes are known wherein in a mixed flow containing various plastic materials, certain kinds of thermoplastic are selectively softened and these are then automatically sorted in further process steps based on the changes made in their physical characteristics.
From GB-A1 313 203 a device is known wherein previously comminuted plastic foam is transported in individual particles on a vibrating surface. Infrared heaters mounted over the conveyor path, heat the conveyed goods, whereby these are brought into their relaxation range and thereby shrunk. The surface of the vibrating conveying device is cooled, in order to prevent an overheating of the surface due to infrared radiation, which would lead to the fusion of the particles of conveyed goods, and an adhesion of the fused particles, which could cause an interruption in the process. Besides the size of the particles to be treated, with this method the process duration is determined by the distance of the IR-radiation elements from the conveyor surface, by the installed cooling capacity and by the conveying speed resulting therefrom. Due to the specific characteristics of a vibratory conveyor, especially an inherent frequency conveyor, combined with the possible overall size of the installation, the transport capacity is limited. A further drawback of this method is that particles to be treated remain with their bottom side on the conveying surface, and these areas are not exposed to a direct IR-radiation. This leads either to an incomplete relaxation of the particles or to a prolongation of the process.
U.S. Pat. No. 3,883,624 describes a process wherein a polystyrene foam, after an introductory comminution, is guided on a metallic conveyor belt through a thermal treatment, whereby the foam shrinks and partially fuses into one layer, becomes brittle after cooling and subsequently is again comminuted into plastifiable particles. Due to the fusion of the polystyrene this process is not suitable for the selective heat treatment of particles in a mixed flow containing also other plastic materials. All foreign matter present in the recycling practice have a negative influence on the process results.
Multiple technical variations of screw conveyors are known which solve the problem of the heat treatment.